1. Field of the Invention
The present invention relates to a code division multiple access communication method using dynamic code assignment. More particularly, the invention pertains to a code division multiple access communication method using dynamic code assignment in which a spectral spread code is dynamically changed to reduce possible interference during communication between a base station and a mobile terminal in an application such as cellular radio communication, and a base station facility for implementing the code division multiple access communication method using dynamic code assignment.
2. Description of the Related Art
In code division multiple access (CDMA) communication, an information signal band is spread out over a wider band for transmission using a code independent of information signals. Since the CDMA provides many advantages such as excellent security of privacy and secrecy of communication, high resistance to interference, adaptability to poor transmission path condition, asynchronous multiple accessibility based on code division, etc., it is expected to be employed extensively as a signaling scheme suitable for mobile communication, e.g., cellular telephone communication.
As a technique of CDMA communication, there is disclosed a DS-CDMA transmission system in Japanese Unexamined Patent Publication No. H10-145282, for example.
In mobile communication which is the principal application field of CDMA communication, it is common practice to divide a geographical control region into cells for implementing cellular communication. For making features of CDMA communication apparent, a multi-sector system which is one of mobile communication cell models will first be explained here.
Referring to FIG. 7, there is shown a schematic diagram for explaining the multi-sector system in cellular radio communication.
A control area called a cell is a geographical area in which communication is set up between a base station and a mobile terminal currently located in the coverage thereof. A multiplicity of cells are arranged in a service area so that each mobile terminal can communicate with at least one of base stations in the service area.
In FIG. 7, each area enclosed in a circle is a cell 42 which is covered by a base station 40.
By using a plurality of antennas each having directivity at the base station 40, the cell 42 is spatially divided into control sub-areas called sectors 41, one of which is shown as a fan-shaped section (hatched) in FIG. 7. Since a directional antenna is used for each of the sectors 41, it is possible to preclude external interference due to other sectors and increase the capacity of communication.
In the cell-sector scheme mentioned above, each mobile terminal such as a cellular telephone belongs to any cell-sector and a call termination service is provided to each mobile terminal upon receipt of a terminating call signal from a base station which covers the cell-sector scheme. In this situation, the mobile terminal periodically checks a common control signal issued by an adjacent base station to always attain base station conditions in the vicinity thereof. When a cell-sector changeover becomes necessary due to movement of the mobile terminal, the current position of the mobile terminal is re-registered in a relevant base station for carrying out a hand-over procedure (which is a link changeover procedure to be performed for switching to another control area).
Then, the following explains codes to be used for spectral spreading in CDMA communication.
In a CDMA system represented by the IS-95 or W-CDMA, the bitwise product or exclusive-OR result of long and short codes in synchronization is used for spread-spectrum coding for downlink transmission from a base station to a mobile terminal.
The functions of the long and short codes are as follows:
In short code assignment, an orthogonal code having a single-symbol cycle is used for identifying each of plural communication channels in a sector and for reducing possible interference between communication channels.
In long code assignment, a long-cycle code having a different phase or a different code sequence per sector or cell is used for identifying each sector or cell.
Table 1 presents a summary of the functional features of these two kinds of codes.
When correlation operation is performed on different short codes for the same long code, orthogonality is attained in coding. By way of contrast, when correlation operation is performed on different short codes for different long codes, orthogonality is not attained in coding. Orthogonalization in coding signifies that mutual interference does not occur in spread-spectrum communications, and non-orthogonalization in coding signifies that mutual interference may take place in spread-spectrum communications.
In CDMA communication, an improvement in its capacity can be made by reducing possible interference. As methods for significantly increasing the capacity of CDMA communication, advanced techniques such as an interference canceller technique and an adaptive array antenna technique have been suggested in publications. The introduction of these interference reduction techniques makes it possible to simultaneously connect more mobile terminals than ever before.
With reference to FIG. 8, a communication system using adaptive array antennas will then be briefly explained.
FIG. 8 is a block diagram of a base station in a communication system wherein adaptive array antennas are employed.
The base station is provided with a plurality of antennas. A signal received by the plural antennas is down-converted through an RF circuit 8 and then fed to a beam former 20, in which a weight value is added so that a signal-to-interference power ratio of the received signal will be maximized. The beam former 20 acts to provide a spatial filter.
More specifically, a main beam is oriented toward a desired user, and a null position of an antenna radiation is formed in the direction of a major interference potential. In this manner of operation, the adaptive array antenna can vary directivity thereof using electrical circuits without having to change physical positioning thereof. The weight-added composite signal is demodulated by a receiver 10 to provide user information.
In downlink transmission, an antenna beam can also be formed in the same manner. For a signal produced by a data generator 6, a beam adapted to the direction of each mobile terminal is formed according to uplink channel (propagation path) information. Thus, a signal is transmitted in a limited range of space. In the 1997 Conference Proceedings of the Institute of Electronics, Information and Communication Engineers B-5-88, for example, there is reported a method for estimating a weight value for downlink transmission according to a correlation matrix attained in uplink transmission. Through formation of a beam by this method, it is possible to substantially suppress interference power in downlink transmission. Therefore, the use of the adaptive array antenna having an advantageous capability of interference suppression allows a substantial increase in the number of simultaneous mobile terminal connections, making it possible to simultaneously connect more mobile terminals than those permissible at a spectral spread rate in a conventional arrangement.
The technique of adaptive array antennas is found in NTT DoCoMo Technical Journal Vol. 5xe2x80x94No. 4, p. 25, for example. Through the use of adaptive array antennas in cellular radio communication, the number of mobile terminals to be simultaneously connected with a base station can be increased in a cell covered by the base station.
As described above, in CDMA communication, an information signal spectrum is spread using a spread code to enable multiple access channel signaling. That is, channels (propagation paths) are separated using different codes for multiple access implementation in CDMA communication.
In other words, the above-stated principle of CDMA communication signifies that the number of usable codes represents the upper limit of multiplexing. In a system such as the IS-95 or W-CDMA, long and short codes are employed and orthogonality of short codes is applied for reduction in inter-channel interference in downlink transmission in particular. Although orthogonalization using short codes is very advantageous for reduction in co-channel interference, there is a problem that the allowable number of orthogonal codes is limited in terms of a short code length. Hence, there may occur a condition that the number of orthogonal short codes becomes insufficient.
As a conventional technique for circumventing this problem, it has been proposed to provide a method of identifying channels by using different-phase long codes for respective mobile terminals. In this method, however, significant interference may occur between channels having different codes, giving rise to a problem of intra-sector interference which would otherwise be non-existent.
As mentioned above, if the number of orthogonal short codes becomes insufficient, it is inevitable to use different long codes. However, since orthogonality is not always attained between different long codes in most cases, inter-channel interference may increase to cause degradation in communication quality.
In the above-cited Japanese Unexamined Patent Publication No. H10-145282, though there is disclosed a technique of selecting an optimum one of some kinds of spread codes according to a transmission speed, no inventive concept is found concerning a method for reducing interference due to insufficiency in the number of spread codes.
On the other hand, in a system using the adaptive array antennas described in the foregoing, a plurality of antennas are used to form each beam for providing RF power toward a desired mobile terminal for communication. Therefore, although interference does not occur among certain mobile terminals in a cell, some mobile terminals spatially located near a base station are likely to be affected by interference. The above-mentioned problem of an increase in interference due to insufficiency in the number of orthogonal codes may be alleviated in terms of probability based on spatial selectivity of the adaptive array antennas.
Nevertheless, since a radio communication system to which the present invention is directed is a mobile communication system, there arises an inherent problem due to time-varying mobility of each terminal. Therefore, even if an optimum code is assigned at the time of link setup, the amount of mutual interference may timewise vary or a source of interference may be changed with time, resulting in degradation in communication quality after a lapse of time. That is, for a mobile terminal, there is a latent problem that an optimum code varies depending on a timewise change in communication conditions.
In a system using sector antennas each having a narrow beam width (having directivity per sector), a mobile terminal frequently undergoes a hand-over procedure for sector-to-sector switching, i.e., it is required to carry out the hand-over procedure through a network for each sector-to-sector switching. This condition implies that not only a mobile terminal being in communication but also a mobile terminal being in a standby state for call reception needs to perform re-registration, thereby posing a problem that a charge-to-charge service life of a battery contained in each mobile terminal is shortened.
It is therefore an object of the present invention to obviate the above-mentioned disadvantages by providing a code division multiple access communication method using dynamic code assignment, wherein an optimum spread code for minimizing inter-channel interference is used according to conditions of communication between a base station and a mobile terminal, and wherein code assignment is dynamically changed for a mobile terminal being in communication through detection of a timing point of code change under initiative control of the base station.
It is another object of the present invention to provide a code division multiple access communication method using dynamic code assignment, wherein code assignment is dynamically changed to reduce inter-channel interference according to conditions such as space correlation and code correlation due to characteristics of adaptive array antennas or sector antennas employed for communication.
It is a further object of the present invention to provide a code division multiple access communication method using dynamic code assignment, wherein a hand-over procedure is carried out under initiative control of a base station for reducing power consumption of a mobile terminal so as to prolong a charge-to-charge service life of a battery contained therein.
It is an additional object of the present invention to provide a mobile communication system comprising a base station control unit connected with a base station, wherein code assignment is dynamically changed without making reference to a network for reducing a networking load in communication between the base station and the base station control unit.
In accomplishing these objects of the present invention and according to one aspect thereof, there is provided a code division multiple access communication method using dynamic code assignment in radio communication based on code division multiple access between a base station and a mobile terminal, comprising: providing an arrangement wherein, under condition that a communication area covered by the base station is variable, while communication is being carried out between the base station and the mobile terminal, a transmitting side notifies a receiving side that a spread code is to be changed, and after notification of spread code reassignment, the spread code is changed so as to reduce interference between a channel used for communication in progress and another channel.
More specifically, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein, at the time of notification of spread code reassignment from the transmitting side to the receiving side, the transmitting side also informs the receiving side of a timing point of spread code reassignment.
Furthermore, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein, at the time of spread code reassignment, a transmission signal spread using a currently assigned code and a transmission signal spread using a new code to be assigned are sent simultaneously, and then when the transmitting side receives an acknowledgment that the receiving side has received the transmission signal spread using the new code to be assigned, the transmitting side stops output of the transmission signal spread using the currently assigned code and sends only the transmission signal spread using the new code.
Further, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein a spread code is formed of the bitwise product or exclusive-OR result of long and short codes, and wherein the transmitting side changes the spread code when the number of short codes to be used for spread coding becomes insufficient.
Still further, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein, at the transmitting side, a space correlation of a plurality of receiving-side channels and a code correlation of a plurality of spread codes used for communication are calculated, each time-space correlation is determined according to the product of space and code correlations, each sum of time-space correlations is calculated with respect to all the inter-channel combinations, a candidate for a new spread code to be assigned is determined according to a minimum-sum time-space correlation, and a procedure for spread code reassignment is carried out to perform communication based on the new spread code.
Still further, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein, at the transmitting side, an interference power level per call is measured, and when the measured interference power level per call exceeds a predetermined threshold, a procedure for spread code reassignment is carried out to change a spread code for a channel corresponding to each call of interest.
Still further from a different viewpoint, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein a base station acts as a transmitting side and a base station control unit is connected with the base station, and wherein, in execution of a procedure for spread code reassignment, a transmission signal is subjected to puncturing at the base station, and information on spread code reassignment is inserted into a free bit space thus produced for transmission to a receiving side.
Still further, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein a receiving side has a common control channel used for identifying a transmitting side and an individual channel dedicated for the receiving side, and wherein the transmitting side radiates a beam for the common control channel in a wider space range than for the individual channel.
In accomplishing the above-mentioned objects of the present invention and according another aspect thereof, there is provided a base station for code division multiple access communication using dynamic code assignment in radio communication between the base station and a mobile terminal under condition that a communication area covered by the base station is variable, comprising: means for notifying a receiving side that a spread code is to be changed while communication is being carried out between the base station and the mobile terminal; and means for changing the spread code after notification of spread code reassignment so as to reduce interference between a channel used for communication in progress and another channel.
More specifically, according to another aspect of the present invention, there is provided a base station for code division multiple access communication using dynamic code assignment, wherein means for informing the receiving side of a timing point of spread code reassignment is included in addition to the means for notifying the receiving side that the spread code is to be changed.
Furthermore, according to another aspect of the present invention, there is provided a base station for code division multiple access communication using dynamic code assignment, further comprising: means for simultaneously sending a transmission signal spread using a currently assigned code and a transmission signal spread using a new code to be assigned at the time of spread code reassignment; and means for, upon receipt of an acknowledgment that the receiving side has received the transmission signal spread using the new code to be assigned, stopping output of the transmission signal spread using the currently assigned code and for sending only the transmission signal spread using the new code.
Still further, according to another aspect of the present invention, there is provided a base station for code division multiple access communication using dynamic code assignment, wherein a spread code is formed of the bitwise product or exclusive-OR result of long and short codes, and wherein there is provided means for changing the spread code when the number of short codes to be used for spread coding becomes insufficient.
Still further, according to another aspect of the present invention, there is provided a base station for code division multiple access communication using dynamic code assignment, further comprising: means for calculating a space correlation of a plurality of receiving-side channels and a code correlation of a plurality of spread codes used for communication; means for determining each time-space correlation according to the product of space and code correlations; and means for performing summation on time-space correlations with respect to all the inter-channel combinations; wherein a candidate for a new spread code to be assigned is determined according to a minimum-sum time-space correlation, and a procedure for spread code reassignment is carried out to perform communication based on the new spread code.
Still further, according to another aspect of the present invention, there is provided a base station for code division multiple access communication using dynamic code assignment, further comprising: means for measuring an interference power level per call, wherein, when the measured interference power level per call exceeds a predetermined threshold, a procedure for spread code reassignment is carried out to change a spread code for a channel corresponding to each call of interest.
Still further from a different viewpoint, according to another aspect of the present invention, there is provided a base station for code division multiple access communication using dynamic code assignment, wherein a base station control unit is connected with the base station, and wherein, in execution of a procedure for spread code reassignment, a transmission signal received by the base station from the base station control unit is subjected to puncturing at the base station, and information on spread code reassignment is inserted into a free bit space thus produced for transmission to a receiving side.
Still further, according to another aspect of the present invention, there is provided a base station for code division multiple access communication using dynamic code assignment, wherein a receiving side has a common control channel used for identifying a transmitting side and an individual channel dedicated for the receiving side, and wherein the base station radiates a beam for the common control channel in a wider space range than for the individual channel.
In accomplishing the above-mentioned objects of the present invention and according to another aspect thereof, there is provided a base station for code division multiple access communication using dynamic code assignment, wherein a spread code is formed of the bitwise product or exclusive-OR result of long and short codes, wherein there is provided means for detecting a bearing of each mobile terminal according to average received signal intensity, and wherein the mobile terminal is notified of spread code reassignment using the means for notification of spread code reassignment, and then a spread code for communication is changed in a fashion that long codes of a mobile terminal group located in the vicinity of the detected bearing are made identical.
More specifically, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein the transmitting side is provided with a plurality of directional antennas capable of radiating radio waves in a limited direction, wherein a region covered by each directional antenna is defined as a physical subsector, a plurality of adjacent physical subsectors are grouped into a wider region in which the same long code is used for communication with communicating parties therein, and the wider region thus formed is defined as a logical subsector, and wherein, at the time of signal reception from each communicating party, under condition that a timewise average value of received signal intensity in an adjacent logical subsector becomes higher than that in a logical subsector currently engaged for communication with the communicating party; or under condition that, at the transmitting side, a space correlation of a plurality of receiving-side channels and a code correlation of a plurality of spread codes used for communication are calculated, each time-space correlation is determined according to the product of space and code correlations, and a value of time-space correlation attained using a long code for an adjacent logical subsector is found to be smaller than that attained using a long code for a logical subsector currently engaged for communication; a procedure for spread code reassignment is carried out to perform communication based on the long code for the logical subsector adjacent to the logical subsector currently engaged for communication.
Still further, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein the transmitting side is provided with a plurality of directional antennas capable of radiating radio waves in a limited direction, wherein a region covered by each directional antenna is defined as a physical subsector, a plurality of adjacent physical subsectors are grouped into a wider region in which the same long code is used for communication with communicating parties therein, and the wider region thus formed is defined as a logical subsector, and wherein, in a situation that the number of short codes becomes insufficient due to an increase in communication traffic in a logical subsector A, a procedure for spread code reassignment is carried out to change a spread code in the following fashion:
(1) if there are short codes available for assignment in a logical subsector B adjacent to the logical subsector A, a physical subsector P which belongs to the logical subsector A and which is positioned on a boundary between the logical subsector A and the logical subsector B is rearranged so that the physical subsector P belongs to the logical subsector B, whereby a long code of the logical subsector B is used for communication with a communicating party in the physical subsector P;
(2) alternatively, if there are no short codes available for assignment in the logical subsector B adjacent to the logical subsector A, a new logical subsector C is formed for the physical subsector P which belongs to the logical subsector A and which is positioned on the boundary between the logical subsector A and the logical subsector B, whereby a long code of the logical subsector C is used for communication with a communicating party in the physical subsector P.
Still further, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein the transmitting side is provided with a plurality of adaptive array antennas, and wherein, under either of the following conditions (1) and (2), the inner product of array weight values is calculated with respect to each communicating party, and a procedure for spread code reassignment is carried out to change a spread code so that the same long code is used for communication with a group of mobile terminals having a high value of inner product:
(1) at the transmitting side, a space correlation of a plurality of receiving-side channels and a code correlation of a plurality of spread codes used for communication are calculated, each time-space correlation is determined according to the product of space and code correlations, each sum of time-space correlations is calculated with respect to all the inter-channel combinations, and a candidate for a new spread code to be assigned is determined according to a minimum-sum time-space correlation for spread code reassignment to perform communication based on the new spread code;
(2) at the transmitting side, an interference power level per call is measured, and when the measured interference level per call exceeds a predetermined threshold, a spread code for a channel corresponding to each call of interest is changed for spread code reassignment.
Still further, according to another aspect of the present invention, there is provided a code division multiple access communication method using dynamic code assignment, wherein a space correlation of a channel used for communication with a mobile terminal A and channels used for communication with other mobile terminals is determined, wherein, with respect to a high-order range xe2x80x9cnxe2x80x9d in terms of space correlation values arranged in descending order (where xe2x80x9cnxe2x80x9d is a predetermined positive integer), comparison is made between a long code in spread coding for the channel used for communication with the mobile terminal A and long codes in spread coding for channels corresponding to the high-order range xe2x80x9cnxe2x80x9d, and wherein a procedure for spread code reassignment is carried out to change a spread code in a fashion that long codes used for communication with other mobile terminals corresponding to the high-order range xe2x80x9cnxe2x80x9d are made identical to the long code used for communication with the mobile terminal A if a mismatch is found therebetween.
In accomplishing the above-mentioned objects of the present invention and according to another aspect thereof, there is provided a base station for code division multiple access communication using dynamic code assignment in radio communication between the base station and a mobile terminal, wherein a base station control unit is connected with the base station, and wherein, at the time of transmission of control information to the mobile terminal, a transmission signal received by the base station from the base station control unit is subjected to puncturing at the base station, and control information is inserted into a free bit space thus produced for transmission to the mobile terminal.
Furthermore, according to another aspect of the present invention, there is provided a mobile terminal for code division multiple access communication using dynamic code assignment in radio communication between a base station and the mobile terminal, wherein the mobile terminal is so arranged as to take control information out of a punctured signal received from the base station.
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.